1. Field of the Invention
Embodiments of the invention relate to a fuel cell system.
2. Description of Related Art
In a fuel cell, flooding sometimes occurs so that the moisture in the fuel cell blocks poles in electrodes and flow channels of separators. If the fuel cell is in a flooding state, the output of the fuel cell may decrease due to the occurrence of an abrupt voltage drop in a high current region. As an art of suppressing the occurrence of such a voltage drop, for example, there is known a technology of resolving flooding by increasing the amount of cathode gas and hence promoting the drainage of water from a fuel cell (e.g., see Japanese Patent Application Publication No. 2013-196782 (JP 2013-196782 A)). There is also known a technology of suppressing the occurrence of a voltage drop by estimating a degree of decrease in voltage of a fuel cell corresponding to an operation state of a motor and limiting an output of the motor in accordance with the degree of decrease in voltage (e.g., see Japanese Patent Application Publication No. 2006-345651 (JP 2006-345651 A)).
However, according to the technology of resolving flooding by increasing the amount of cathode gas, a predetermined time is required from a request to increase the amount of cathode gas to the resolution of flooding. In consequence, for example, if a request to increase the output of the fuel cell is made in the flooding state, the operation state of the fuel cell shifts to the high current region to cause a voltage drop before the resolution of flooding, and as a result, the output of the fuel cell may decrease.
According to the technology of estimating the degree of decrease in voltage and limiting the output of the motor, the degree of decrease in voltage at present is estimated based on an actually measured output voltage value of the fuel cell at present. Therefore, for example, if the operation state of the fuel cell shifts from a low current region to the high current region in a short period, the estimation of the degree of decrease in voltage cannot cope with the shift in the operation state of the fuel cell. As a result, the degree of decrease in voltage exceeds a degree to which it should be limited, so a voltage drop may occur in the high current region. If so, the voltage may be controlled in such a manner as to return to a pre-voltage drop state, based on the later estimated degree of decrease in voltage, and the convergence performance of the output of the fuel cell may decrease.